Field of the Invention
This invention relates generally to a data transfer system and related transfer method, and more specifically to a data transfer system and related method for transferring data between a main memory and a plurality of disk drives which may be asynchronously rotated.
II. Backqround Information
Conventional memory systems for large computers typically include a main memory and several magnetic disks on disk drives. Main memory comprises the fast portion of a typical large computer memory system. Accordingly, data which must be manipulated is stored in main memory. Afterwards, the manipulated data is stored in the magnetic disks. For example, image data is first received from a data input source, manipulated in main memory, and stored in magnetic disks from which the data may be returned to main memory where the data is readied for image display on a cathode ray tube display device or the like.
Disk drives typically comprise several magnetic disks which are mounted horizontally on a rotating shaft. The shaft rotates the several disks adjacent read/write heads which write data into the surfaces of the rotating disks or read data from the surfaces of the disks. For example, five such disks may be stacked in a single shaft and rotated at a constant speed.
Each of the disks has two surfaces, an upper and a lower surface, each rotating adjacent an associated head. The surfaces of the disks include concentric circular tracks, referred to as cylinders. Each of the disks on a particular disk drive have the same number of cylinders, and the heads adjacent the disks of the disk drive at any given time are all situated adjacent cylinders which are at the same radial distance from the center of the disk. The cylinders of the disk surfaces are divided into discrete radial portions referred to as sectors. The sectors have a given memory capacity specified by byte size. Typically, a disk surface may have 823 concentric circular tracks or cylinders and each cylinder may have 35 sectors each having a memory capacity of 512 bytes.
Data to be stored on the magnetic disks of the disk drives is transferred between or, more specifically, written from the main memory to the sectors of the cylinders of the disks for later retrieval when needed. The stored data, when needed, is transferred between or, more specifically, read from the sectors of the cylinders to the main memory. The data is then available for use from the main memory by units such as an image processing unit. Generally, the time taken for reading data from, and writing data into disks is fairly long in comparison with the time required to transfer data between a head and the main memory.
A disk data transfer system is used to transfer data between the main memory and the disk drives. The typical data transfer system includes a transfer control unit, disk drive interfaces, and a main memory access unit which are implemented so as to transfer disk data quickly between corresponding memory locations and cylinder sectors.
Typically, the transfer control unit receives transfer commands which indicate the direction of the transfer (from disk drives to main memory (read) or from main memory to disk drives (write)). The transfer control unit also receives control signals indicating the beginning or leading main memory location for the transfer, the leading disk drive sector for the transfer and the number of sectors of data for the transfer. The transfer control unit processes the leading sector and the number of sectors to determine the leading sectors and number of sectors for each of the disk drives.
For each disk drive there exists a disk drive interface. Each disk drive interface interconnects a disk drive with the transfer control unit. Each disk drive interface receives the leading sector and the number of sectors for that disk drive with which that interface is connected. The disk drive interface polls that disk drive to determine if the sector of the cylinder which is currently rotating adjacent the recording head, that is, the current sector, matches the leading sector as received from the transfer control means. If the current sector is not the same as the leading sector, the disks continue to rotate, uneventfully. When the current sector matches the leading sector, a data transfer is initiated in the transfer direction as received by the transfer control unit. Data transfer continues for each drive until the number of sectors to be transferred between main memory and each disk drive have been transferred.
The disk data is typically held by the interface as the data is transferred between the disk drive and the main memory.
The main memory access unit links the disk drive interfaces to the main memory so that the disk data can be transferred between the disk drive interfaces and the main memory. The main memory access transfers disk data to and from main memory locations according to the leading main memory location for the transfer. The leading main memory location is provided by the transfer control means. All data is received by the main memory access unit, from the disk interfaces for a read transfer or from the main memory for a write transfer, in the same order as the data is to be received by the main memory or the disk interfaces, respectively.
In a conventional memory system, the disk drives are arranged in parallel The parallel arrangement of the disk drives permits a disk data transfer system to achieve improved data transfer speed. Additionally, data transfer system may be implemented using asynchronously rotating disk drives as disclosed in U.S. Pat. No. 4,590,559 to Baldwin et al., since synchronizing rotation of disk drives is difficult to achieve and results in lowered data transfer speed.
As has been described, the known disk data transfer systems do not initiate data transfer until the leading sector is under the recording head.
The effect of awaiting the leading sector in order to initiate data transfer to ensure the proper ordering of disk data is to cause the transfer system to ignore transfer sectors (i.e., sectors to which or from which data is to be transferred) rotating under the recording heads before the leading transfer sector. This arrangement impedes the speed of data transfer.
Accordingly, an object of the present invention is to provide a disk data transfer system and related method which may more quickly and efficiently transfer data between main memory and the disk drives.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.